1. Field of Invention
This invention relates to ion cyclotron resonance (ICR) mass spectrometers (MS), preferably to Fourier transform ICR (FTICR) MS, in which the detection of repetitive oscillations of clouds of ions is performed at fundamental or overtone frequencies and the analysis of those frequencies allows a mass spectrum to be determined.
2. Discussion of the Background
In a cyclotron resonance (ICR) mass spectrometer (MS) the mass-specific cyclotron motions of the ions in a magnetic field are detected as image currents induced by the ions in detection electrodes.
A discrete Fourier transformation (DFT), a form of Fourier transformation (FT) used for discrete signals, is usually used to convert the detected currents into a spectrum of the ion oscillation frequencies which is then converted into a mass spectrum using a mathematical calibration procedure that typically accounts for numerous distortions to the frequency spectra caused, for example, by superimposed magnetron motion or ion space charge. In addition to DFT, and particularly fast Fourier transformation (FFT), other types of mathematical transformations (for example, wavelet and chirplet transforms, shifted-basis techniques, or filter-diagonalization method) can be used to convert the time domain of the detected image currents into the frequency spectrum.
Typically, in ICR mass spectrometers the detection of fundamental frequencies of ion oscillations is performed. The problems associated with the detection of the fundamental frequencies are widely known and typically include space charge effects, non-ideality of the magnetic and electric fields used, and distortions in the detection system. The latter usually results in observation of harmonic frequencies (multiples of the fundamental one) in the frequency spectra that can result in observation of “ghost” peaks in the mass spectrum. The problem of the “ghost” peaks in ICR-MS based on the detection of the ion fundamental frequencies is usually solved by designing a detection system as close to an ideal one (i.e., one having ideal sine waveform response on the system's fundamental oscillation) as possible. In another method, software processing is used to remove the harmonics from the frequency spectrum in a regular FTICR-MS. See Franzen and Michelmann, US Pat. Appl. 2009/0084949, the entire contents of which are incorporated herein by reference.
In addition to ICR mass spectrometers based on the detection of the ion fundamental frequencies, there is another type of mass detection based on the detection of the ion oscillation overtone frequencies. Overtone frequencies are typically a multiple of the fundamental frequency. There is a difference between oscillation harmonics and oscillation overtones. Harmonics are usually observed due to system non-ideality (for example, due to deviation of system potential energy from harmonic one) or distortions in signal processing (like clipping sine waveforms). In contrast, overtones can be observed even in the absence of non-ideal factors and signal distortions. Both overtones and harmonics relate to a system fundamental oscillation (which can be thought of as system small oscillation at the lowest characteristic frequency). Since both fundamental and overtone oscillations can be observed at ideal conditions (i.e., at ideal harmonic potential and without signal distortions), the overtone observation is determined by factors other than system non-idealities, particularly by methods of oscillation generation or detection. For example, in a guitar, overtones can be generated by a special plucking of a guitar string. In quantum mechanics, an overtone excitation of a harmonic oscillator corresponds to its excitation to the energy level corresponding to more than one quantum.
“Synchronized” magnetron motion (described below) is responsible for the appearance of the side-shifted peaks, and this type of ion motion is very difficult to avoid in a typical ICR experiment. The relative intensity of subharmonics, harmonics, and side-shifted peaks in ICR-MS spectra increases significantly with the increase of the overtone order on which detection is performed. Therefore, the same magnitude of the “synchronized” ion magnetron motion and degree of imperfections in the detection system in a conventional detection scheme and the one with overtone detection will result in significantly higher level of the subharmonics and side-shifted peaks in the latter one compared to the level of harmonics in the former, conventional detection system. The problem of the “ghost” peaks (i.e., subharmonics and side-shifted peaks) is significantly exacerbated in the overtone detection schemes compared to the problem of harmonics in conventional detection of the ion fundamental oscillations.
As mentioned above, there are two primary conventional ways to fight the “ghost” peaks in a regular ICR-MS with the detection of the fundamental oscillations. The preferred one is based on optimizing ICR-MS hardware by designing an “ideal” detection system that does not generate harmonic frequencies in the detected signal. The other one is based on software processing to remove the harmonics in the detected frequency spectrum.
The following references are incorporated by reference herein in their entirety and describe background technology:
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